Audio reproducing apparatus

ABSTRACT

An audio signal processing apparatus and method for extracting components from an input signal, generating additional components, combining components, and level-controlling components. An audio signal processing apparatus may include a harmonic overtone adder and an equalizer. A harmonic overtone adder may include a high-pass filter, a low-pass filter, an harmonic overtone generator, and a combining unit. An equalizer may include a level detector and a gain controller.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention is a continuation of U.S. Ser. No. 11/904,451,filed Sep. 27, 2007, which claims the priority benefit of Japanesepatent application number 2006-283532, filed in the Japanese PatentOffice on Oct. 18, 2006, which is hereby incorporated by reference tothe maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an audio reproducing apparatus.

2. Description of the Related Art

Small loudspeakers are employed in minicomponent stereo sets andflat-screen television receivers. In such applications, the enclosure ofthe loudspeaker (speaker box) is accordingly small. The resonancefrequency f0 of the loudspeaker is as high as or higher than 100 Hz.

An audio signal having the resonance frequency f0 or lower may besupplied to the loudspeaker. With the frequency being lowered, thefundamental component is increasingly lowered while distorted components(harmonic components) sharply increases in the sound output pressurethereof.

Audio apparatuses having small loudspeakers cannot sufficientlyreproduce low-frequency component lower than the resonance frequency f0of the loudspeaker.

Two methods may be contemplated in the reproducing of the audio signal:

(1) an equalizer is used to boost the low-frequency component, and

(2) a harmonic overtone component of the low-frequency component isoutput to achieve a low-frequency sound effect.

The method (1) provides the low-frequency sound effect by reinforcingthe frequency component in the resonance frequency f0 band of theloudspeaker.

The method (2) takes advantage of the hearing of humans. Morespecifically, the sound of each musical instrument is composed of afundamental component and harmonic components, and the ratio of thefundamental component to the harmonic components determines the tone ofthe sound. It has been psychoacoustically proved that if a sound withoutthe fundamental component but with the harmonic components thereof isoutput humans hear as if the fundamental component is also output. Themethod (2) is based on such a human hearing property.

FIG. 14 illustrates an audio apparatus. A loudspeaker 5 is used toimprove the low frequency sound effect. An audio signal S1 is suppliedto a high-pass filter 2 at an input terminal 1. As shown in FIG. 15A, amiddle to high-frequency component equal to or higher than the resonancefrequency f0 of the loudspeaker 5 is extracted and supplied to an adder3. The audio signal S1 at the terminal 1 is supplied to a band-passfilter 7. As shown in FIG. 15B, a low-frequency component S7 fallingwithin a frequency band from f0/2 to f0 is extracted and supplied to apitch shifter 8.

The pitch shifter 8 doubles the frequency of the supplied low-frequencycomponent S7. As shown in FIG. 15C, a frequency multiplied component S8within a band from f0 to 2f0, namely, the low-frequency component S8, isoutput.

The low-frequency component S8 is supplied to the adder 3 to be added tothe middle to high-frequency component S2. The adder 3 outputs an audiosignal S3 with the low-frequency component S8 as the low-frequencycomponent S7 reinforced as shown in FIG. 15D. The audio signal S3 isoutput to the loudspeaker 5 via a power amplifier 4. The loudspeaker 5thus emits an acoustic sound having frequency characteristics of FIG.15D, namely, an acoustic sound with the low-frequency component S8 asthe reinforced low-frequency component S7.

The sound of the low-frequency component S7 is not output from theloudspeaker 5 and corresponds to the fundamental component. Thecorresponding low-frequency component S8 is output from the loudspeaker5. A listener hears as if the low-frequency component S7 is actuallyoutput. Even with the small loudspeaker 5, the low frequency soundeffect is thus provided.

It is generally said that humans suffer from no unpleasant hearingimpression on the low-frequency component S8 lower than 200 Hz even ifthe frequency multiplied component S5 is generated by multiplying thelow-frequency component S7.

Japanese Unexamined Patent Application Publication No. 8-213862discloses one such technique.

SUMMARY OF THE INVENTION

The method (1) of boosting the low-frequency component is notappropriate for the small loudspeaker that is originally unable toreproduce the low-frequency component. If an equalizer with a boostamount thereof fixed is used, signals are clipped, leading to noise anddistortion. Supplying a deep bass component to the small loudspeaker isnot preferable.

The method (2) of adding the harmonic overtone is effective with thesmall loudspeaker since the low-frequency component is not supplied tothe loudspeaker 5. The low-frequency component S8 is similar to aharmonic component of the band-pass filter 7. If the low-frequencycomponent S8 is increased in amount to achieve the low frequency soundeffect, the degree of distortion is also increased. If the low-frequencycomponent S8 is reduced in amount to lower the degree of distortion, thelow frequency sound effect becomes insufficient. There is a trade-offbetween the low frequency sound effect and the degree of distortion.

It is thus desirable to overcome such a problem.

An audio signal processing apparatus includes a harmonic overtone adderand an equalizer. The harmonic overtone adder includes a high-passfilter for extracting from an audio signal a frequency component equalto or higher than a first predetermined frequency, a filter forextracting from the audio signal a frequency component equal to or lowerthan half a second predetermined frequency, an harmonic overtonegenerator for generating a frequency-doubled harmonic overtone componentfrom an output from the filter, and a first combining unit for combiningthe frequency component output from the high-pass filter and theharmonic overtone component output from the harmonic overtone generator.The equalizer includes a level detector for detecting a level of anovertone component contained in an output from the first combining unit,a gain controller for controlling dynamically the level of the harmonicovertone component contained in the output from the first combining unitbased on a detection output from the level detector, and a secondcombining unit for combining the output from the first combining unitwith the harmonic overtone component output from the gain controller.

In accordance with embodiments of the present invention, if thelow-frequency component of the audio signal is lower than the firstpredetermined frequency, the harmonic overtone of the low-frequencycomponent is output. The harmonic overtone provides the low frequencysound effect. Since the level of the harmonic overtone is dynamicallyvaried, a crisp low frequency sound effect is obtained while the degreeof distortion in the output sound is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a harmonic overtone adder in accordancewith one embodiment of the present invention;

FIGS. 2A-2F illustrate frequency characteristics of the harmonicovertone adder in accordance with one embodiment of the presentinvention;

FIG. 3 is a waveform diagram of the harmonic overtone adder;

FIG. 4 is a control characteristic chart of the harmonic overtone adder;

FIG. 5 is a block diagram of another harmonic overtone adder inaccordance with one embodiment of the present invention;

FIG. 6 is a block diagram illustrating yet another harmonic overtoneadder in accordance with one embodiment of the present invention;

FIGS. 7A-7C illustrates frequency characteristics of the harmonicovertone adder of FIG. 6;

FIGS. 8A and 8B illustrate frequency characteristics of the harmonicovertone adder of FIG. 6;

FIG. 9 illustrates still another harmonic overtone adder in accordancewith one embodiment of the present invention;

FIGS. 10A and 10B illustrate frequency characteristics of the harmonicovertone adder of FIG. 9;

FIGS. 11A-11C are waveform diagrams of the harmonic overtone adder;

FIGS. 12 illustrates a pitch shifter in the harmonic overtone adder;

FIGS. 13A and 13B are waveform diagrams of the patch shifter;

FIG. 14 illustrates a harmonic overtone adder as a known art; and

FIGS. 15A-15D illustrate frequency characteristics of the known harmonicovertone adder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a harmonic overtone adder 10 in accordance with oneembodiment of the present invention. A small loudspeaker 33 provides animproved low frequency sound effect. Let f0 represent a resonancefrequency of the loudspeaker 33. The resonance frequency f0 is 100 Hz orlower. Let f1 represent a frequency upper limit below which a signalobtained by frequency multiplying a fundamental frequency signal causesno unpleasant hearing impression. The frequency upper limit f1 is afrequency obtained by frequency multiplying a fundamental frequency of asignal. The frequency upper limit f1 is about 200 Hz. Here, f0=f1/2 (orf0≦f1/2). In the case of a two-channel stereophonic system ormulti-channel stereophonic system, each channel has the structure ofFIG. 1.

An audio signal S11 to be reproduced is supplied to a harmonic overtoneadder 10. A harmonic overtone component providing a low-frequency soundeffect is thus added to the audio signal S11. The audio signal S11 issupplied to a high-pass filter 12 via an input terminal 11. As shown bya solid line in FIG. 2A, a low-frequency component S12 equal to orhigher than the resonance frequency f0 of the loudspeaker 33, namely 100Hz, in this case is extracted and then supplied to an adder 13. Theaudio signal S11 is supplied from the input terminal 11 to a low-passfilter 14. As shown by a broken line in FIG. 2A, a low-frequencycomponent S14 equal to or lower than the resonance frequency f0 of theloudspeaker 33, namely 100 Hz, in this case is extracted by the low-passfilter 14. The low-frequency component S14 is then supplied to anattenuator 15. The attenuator 15 adjusted the low-frequency componentS14 to a predetermined level and the resulting level adjusted componentis supplied to the adder 13.

The audio signal S11 is supplied from the input terminal 11 to aband-pass filter 16. As shown by a solid line in FIG. 2B, alow-frequency component S16 falling within a frequency band f0/2 tof1/2, namely, 50 Hz to 100 Hz, is extracted. The low-frequency componentS16 is then supplied to a pitch shifter 17.

One example of the pitch shifter 17 will be described later. The pitchshifter 17 doubles the frequency of the supplied low-frequency componentS16. As shown by a broken line in FIG. 2B, a frequency doubled, harmonicovertone component S17 is extracted. Since the low-frequency componentS16 falls within the frequency band f0/2-f1/2, the frequency band of theharmonic overtone component S17 becomes f0-f1. The frequency-doubledharmonic overtone component S17 is adjusted to a predetermined level byan attenuator 18 and the level-adjusted harmonic overtone component S17is then supplied to the adder 13.

As shown in FIG. 2C, the adder 13 outputs an audio signal S13. The audiosignal S13 is obtained by adding the low-frequency component S14 and thefrequency-doubled harmonic overtone component S17 at predeterminedratios to the middle to high-frequency component S12.

The audio signal S13 is supplied to a gain-controlling type equalizer20. The equalizer 20 performs an equalization operation accounting forlow frequency sound effect and distortion effect. The audio signal S13is supplied to an adder 21 and a band-pass filter 22. As shown in FIG.2D, a low-frequency component S22 falling within a frequency band f0-f1,namely, 100 Hz-200 Hz, is extracted. The low-frequency component S22 issupplied to the adder 21 via a gain controller 23 to be discussed later.

As shown in FIG. 2D, the low-frequency component S22 contains alow-frequency component S121 of the middle to high-frequency componentS12 (FIG. 2A) and the harmonic overtone component S17. A signalcomponent S23 as an output signal of the gain controller 23 is obtainedby level controlling the low-frequency component S22, and contains thelow-frequency component S121 and the pitch shifter 17.

The low-frequency component S22 is supplied from the band-pass filter 22to a level detector 24. As represented by a solid wave line in FIG. 3,one cycle of the low-frequency component S22, i.e., a duration Tx from areversal from negative to positive to a next reversal from negative topositive is determined as one period. A peak level V22 (absolute value),detected within one period Tx, is referred to as a detected signal S24(absolute value). The detected signal S24 is supplied to the gaincontroller 23 as a gain control signal.

FIG. 4 illustrates control characteristics of the gain controller 23.The abscissa represents the input level of the low-frequency componentS22 supplied to the gain controller 23, namely, the peak level V22 ofthe band-pass filter 22 within one period Tx from which the signal S24is detected. The ordinate represents an output level V23 of alow-frequency component S23 output from the gain controller 23. A brokenline B represents characteristics with gain being 1 regardless of theinput level V22 (peak level) for reference only in FIG. 4.

Control characteristics of the gain controller 23 is represented by lineA. Let VLM represent a predetermined upper limit and VTH represent apredetermined threshold level (VLM>VTH). V23=VMAX holds if V22≧VLM. V23is in linear proportion to level V22 with relationship V22<VLM held.Gain is greater than 1 if relationship VTH<V22<VLM holds. Gain is 1 ifrelationship V22=VTH holds. Gain is smaller than 1 if relationshipV22<VTH holds.

The gain controller 23 level controls the low-frequency component S22 ona per period Tx in accordance with the detected signal S24 and thecontrol characteristics A. The peak level V22 cannot be known until oneperiod Tx is completed. For simplicity, it is assumed the peak level V22within the one period Tx can be detected at the start of the period Tx.For detection and control, the low-frequency component S22 to be levelcontrolled is pre-delayed to be synchronized with the correspondingdetected signal S24.

The adder 21 adds the low-frequency component S23 level controlled bythe gain controller 23 to the audio signal S13 from the adder 13. Asshown in FIG. 2E, the adder 13 outputs an audio signal S21 composed ofthe frequency components S14, S121, S17 and S12. The audio signal S21 isthen supplied to a low-frequency cutoff filter 31. As shown in FIG. 2F,the low-frequency cutoff filter 31 removes a deep bass componentdetrimental to the small loudspeaker 33, i.e., the low-frequency cutofffilter 31 outputs an audio signal S31. Although almost no standard lowfrequency sound is output from the loudspeaker 33, a low-frequencycomponent becoming a distorted component (harmonic component) could beoutput. The low-frequency cutoff filter 31 removes such a low-frequencycomponent. The low-frequency cutoff filter 31 is then supplied to theloudspeaker 33 via a power amplifier 32.

With the above-described arrangement, the audio signal S31 of FIG. 2F isoutput with the audio signal S11 input to the input terminal 11. Theaudio signal S31 is then supplied to the loudspeaker 33, and theresulting sound is output from the loudspeaker 33. As shown in FIG. 2F,the audio signal S31 supplied to the loudspeaker 33 contains theharmonic overtone component S17 twice as high in frequency as thelow-frequency component S16 (FIG. 2B).

Although almost no sound of the fundamental component lower than theresonance frequency f0 is output from the loudspeaker 33, the harmonicovertone component S17 twice as high as the low-frequency component S16is output. A listener may hear the sound as if the sound lower than theresonance frequency f0 is output. Even with the small loudspeaker 33,the low frequency sound effect is provided.

The gain controller 23 level controls the low-frequency component S22containing the harmonic overtone component S17 to the low-frequencycomponent S23. Since the low-frequency component S23 has controlcharacteristics as shown in FIG. 4, the output low-frequency componentS23 has the waveform (level) as represented by a broken line in FIG. 3.If the peak level V22 of the low-frequency component S22 within the oneperiod Tx is lower than the threshold level VTH, the output level V23 ofthe low-frequency component S23 becomes smaller than the originalmagnitude as represented by an arrow C in FIG. 4. If the peak level V22of the low-frequency component S22 within the one period Tx is higherthan the threshold level VTH, the output level V23 of the low-frequencycomponent S23 becomes larger than the original magnitude as representedby an arrow D in FIG. 4.

As represented by the broken line in FIG. 3, the output level V23 of thelow-frequency component S23 output from the gain controller 23 becomesmore smaller throughout one period Tx during which the peak level V22 issmaller than the threshold level VTH, and becomes more larger throughoutone period Tx during which the peak level V22 is larger than thethreshold level VTH. Since the harmonic overtone component S17 containedin the low-frequency component S23 dynamically changes in level in thesame way, a crisp low frequency sound effect is reached while distortioneffect is restricted.

Since part of the low-frequency component S14 contained in the audiosignal S11 still remains as shown in FIGS. 2A and 2F, the low frequencysound effect becomes natural. A deep bass component detrimental to thesmall loudspeaker 33, out of the low-frequency component S14, is removedby the low-frequency cutoff filter 31, and outputting a large amount ofdistorted component (harmonic component) is avoided.

In the above discussion, the low-frequency component S22 output from theband-pass filter 22 is supplied to the detector 24. As represented by abroken line in FIG. 1, the audio signal S13 output from the adder 13 maybe supplied to the detector 24 to obtain the detected signal S24.

FIG. 5 illustrates a harmonic overtone adder 10 in accordance with asecond embodiment of the present invention. In the harmonic overtoneadder 10, the band-pass filter 16 is removed, and the low-frequencycomponent S14 output from the low-pass filter 14 is supplied to thepitch shifter 17. The band-pass filter 22 is replaced with a low-passfilter 25 having the upper limit frequency f1 as a cutoff frequency. Therest of the harmonic overtone adder of FIG. 5 is identical to theharmonic overtone adder of FIG. 1.

The harmonic overtone component S17 of the low-frequency component S14is output as a sound. The listener thus hears the sound as if the soundbelow the resonance frequency f0 is also output. Even the smallloudspeaker 33 provides the low frequency sound effect.

The gain controller 23, having the control characteristics of FIG. 4,level controls the low-frequency component S22 containing the harmonicovertone component S17 to the low-frequency component S23. The harmonicovertone component S17 contained in the low frequency component S23output from the gain controller 23 dynamically changes the levelthereof. A crisp low frequency sound effect results while distortioneffect is controlled.

As represented by a broken line in FIG. 5, the audio signal S13 outputfrom the adder 13 is supplied to the detector 24, and the detector 24outputs the detected signal S24.

FIG. 6 illustrates a first modification of the harmonic overtone adder10. As shown in FIG. 6, part of the harmonic overtone adder 10 isidentical to the harmonic overtone adder 10 of FIG. 1. As shown in FIG.7A, a high-pass filter 12 and the low-pass filter 14 respectivelyextract the middle to high frequency component S12 and the low-frequencycomponent S14 from the audio signal S11, and then supply thesecomponents to the adder 13.

As shown in FIG. 7B, the band-pass filter 16 extracts the low-frequencycomponent S16 falling within a range of f0/2 to f1/2, and the pitchshifter 17 doubles the frequency of the low-frequency component S16 tothe harmonic overtone component S17. The harmonic overtone component S17is supplied to the adder 13 via the attenuator 18.

The audio signal S11 is supplied to a band-pass filter 46 via the inputterminal 11. As shown in FIG. 7B, a low-frequency component S46 fallingwithin a frequency range f0/4-f1/4 is thus extracted. The low-frequencycomponent S46 is frequency multiplied by a pitch shifter 47 into aquadrupled frequency, harmonic overtone component S47. The harmonicovertone component S47 is supplied to the adder 13 via an attenuator 48.

As shown in FIG. 7C, the adder 13 adds to the middle to high-frequencycomponent S12, the low-frequency component S14, the frequency-doubled,harmonic overtone component S17 and the frequency-quadrupled, harmonicovertone component S47 at predetermined ratios through the attenuators15, 18 and 48, thereby outputting the resulting audio signal S13.

The subsequent process is identical to the process discussed withreference to FIG. 1. The audio signal S13 is supplied to the loudspeaker33 via the equalizer 20, the low-frequency cutoff filter 31 and thepower amplifier 32, though the discussion thereof is omitted herein.

With this arrangement, the loudspeaker 33 emits almost no sound of thefundamental component below the resonance frequency f0, but emits thesound of the harmonic overtone component S17 and the harmonic overtonecomponent S47. The listener thus hears the sound as if the sound lowerthan the resonance frequency f0 is output. Even with the smallloudspeaker 33, the low frequency sound effect is achieved.

If the low-frequency component S16 (S46) is 35 Hz as shown in FIG. 8A,the harmonic overtone component S17 (represented by an arrow-headedbroken line) obtained by doubling the frequency of the low-frequencycomponent S16 has a frequency of 70 Hz, and the loudspeaker 33 is stillunable to reproduce the harmonic overtone component S17 as representedby frequency characteristics F33 of the loudspeaker 33.

In the harmonic overtone adder 10 of FIG. 6, the low-frequency componentS16 having a frequency of 35 Hz, namely, the low-frequency component S46is supplied to the pitch shifter 47 via the band-pass filter 46. Thepitch shifter 47 frequency quadruples the low-frequency component S16 tothe frequency-quadrupled, harmonic overtone component S47 (arrow-headedsolid line) having a frequency of 140 Hz. The harmonic overtonecomponent S47 is supplied to the adder 13. With the low-frequencycomponent S16 having a frequency of 35 Hz, the frequency quadrupled,harmonic overtone component S47 results in a low frequency soundcorresponding to the harmonic overtone component S17.

As shown in FIG. 8B, the low-frequency component S46 (S16) might have afrequency of 60 Hz. If the low-frequency component S46 is quadrupled infrequency to the harmonic overtone component S47 (as represented by anarrow-headed broken line), the resulting frequency is 240 Hz. Theresulting frequency of 240 Hz is above the frequency upper limit f1(≈2200 Hz) in the addition of the harmonic overtone. If the resultingharmonic overtone component S47 is supplied to the loudspeaker 33, anoutput sound results in an unpleasant hearing impression in human ears.

The low-frequency component S46 might have a frequency of 60 Hz in theharmonic overtone adder 10 of FIG. 6. The band-pass filter 46, namely,the low-frequency component S16 is supplied to the pitch shifter 17 viathe band-pass filter 16. The pitch shifter 17 frequency doubles thelow-frequency component S16 to the frequency-doubled, harmonic overtonecomponent S17 (represented by an arrow-headed solid line) having afrequency of 120 H. The resulting harmonic overtone component S17 issupplied to the adder 13. Even when the low-frequency component S46 hasa frequency of 60 Hz, the frequency-doubled, harmonic overtone componentS17 results in a low frequency sound corresponding to the low-frequencycomponent S46.

FIG. 9 illustrates a second modification of the harmonic overtone adder10. As the harmonic overtone adder 10 of FIG. 6, the harmonic overtoneadder 10 of FIG. 9 adds the frequency-quadrupled, harmonic overtonecomponent S47 if the fundamental frequency component is low. Theharmonic overtone components S17 and S47 respectively output from thepitch shifters 17 and 47 are supplied to an adder 19 via the attenuators18 and 48. As shown in FIG. 10A, a harmonic overtone component S19containing the harmonic overtone components S17 and S47 is extracted andthen supplied to a low-pass filter 49.

The low-pass filter 49 has frequency characteristics F49 of FIG. 10B.The low-pass filter 49 has the frequency upper limit f1 as the cutofffrequency thereof at which the input signal is almost cut off.

The low-pass filter 49 outputs, out of the low-frequency component S19,a frequency-doubled and frequency-quadrupled harmonic overtone componentS49 (hatched area) which causes no unpleasant hearing impression. Theharmonic overtone component S49 is supplied to the adder 13.

The rest of the harmonic overtone adder of FIG. 5 is identical to theharmonic overtone adder of FIG. 1. The audio signal S13 is supplied tothe loudspeaker 33 via the equalizer 20, the low-frequency cutoff filter31 and the power amplifier 32, though such path is not shown.

As the harmonic overtone adder 10 of FIG. 6, the pitch shifters 17 and47 output the frequency-doubled harmonic overtone component S17 and thefrequency-quadrupled harmonic overtone component S47 by doubling thefrequency of the low-frequency component S16 and by quadrupling thefrequency of the low-frequency component S46, respectively. The listenermay hear the sound as if the sound lower than the resonance frequency f0is emitted. Even with the small loudspeaker 33, the low frequency soundeffect is achieved.

The low-pass filter 49 lowers more the level of the low-frequencycomponent S19 output from an adder 19 as frequency becomes closer to thefrequency upper limit f1. Even if the low-frequency component S19contains a frequency component close to or even above the frequencyupper limit f1, an unpleasant hearing impression is controlled. A lowfrequency sound effect is thus provided even within the frequency rangef0>f1/2 without any unpleasant hearing impression.

The harmonic overtone component S17 twice in frequency the low-frequencycomponent S16 and the pitch shifter 47 four times in frequency thelow-frequency component S46 are produced as shown in FIGS. 11A-11C. Asshown in FIG. 11A, digital data DA for digital-to-analog converting oneperiod of a sinusoidal signal SA is now stored on a memory. Each soliddot symbol “•” represents a sampling point, and one sample is stored atone corresponding address. A duration TA represents one sample cycle ofthe sinusoidal signal SA, and a duration 1/fc is one sample period.

If the digital data DA is read at a clock frequency fc identical to awrite clock, one cycle of the sinusoidal signal SA can be read for theduration TA.

The digital data DA may be read from one at every two addresses at theclock frequency fc identical to the write clock, and the readingoperation is repeated twice as shown in FIG. 11B. A sinusoidal signal SBof two cycles twice in frequency the sinusoidal signal SA is obtainedfor the duration TA. More specifically, during the duration TA, theharmonic overtone component SB twice in frequency the sinusoidal signalSA is obtained.

The digital data DA may be read from one at every four addresses at theclock frequency fc identical to the write clock, and the readingoperation is repeated four times as shown in FIG. 11C. A sinusoidalsignal SC of four cycles four times in frequency the sinusoidal signalSA is obtained for the duration TA. More specifically, during theduration TA, the harmonic overtone component SC four times in frequencythe sinusoidal signal SA is obtained.

The pitch shifters 17 and 47 have a structure as shown in FIG. 12. FIG.12 illustrates a memory 17M having large number of addresses (largecapacity) composed of a ring buffer. The low frequency component S16 mayhave a waveform of FIG. 13A, digital data D16 may be obtained byanalog-to-digital converting the waveform, and fc may represent asampling frequency (clock frequency).

Let tx represent the timing at which the polarity of the digital dataD16 (low-frequency component S16) is reversed from negative to positive,and Tx represent a duration from one tx to the next tx, namely, oneperiod of the low-frequency component S16.

As shown in FIG. 12, the digital data D16 is supplied to the memory 17Mvia an input terminal 17A. As shown in FIG. 13A, the digital data D16 iswritten on the addresses of the memory 17M every one sample. Theduration Tx of FIGS. 13A and 13B correspond to the duration TA of FIG.11A. FIG. 13A corresponds to FIG. 11A.

At the same time as the digital data D16 is written on the memory 17M,the digital data D16 is read from the memory 17M. For simplicity ofexplanation, the duration Tx for the write operation equals the durationTx for the read operation in FIGS. 13A and 13B.

The read operation from the pitch shifter 17 is performed in the samemanner as described with reference to FIG. 11B. More specifically, thedigital data D16 is read from one at every two addresses at the clockfrequency fc equal to the one for the write operation. During theduration Tx, the read operation is repeated twice. Read digital data D17is digital-to-analog converted, and the harmonic overtone component S17twice in frequency the original low-frequency component S16 is obtained.

Similarly, the low-frequency component S46 is analog-to-digitalconverted and written on the memory 17M. The written data is then readas shown in FIG. 11C. The digital data D16 is read from one at everyfour addresses at the clock frequency fc equal to the one for the writeoperation. During the duration Tx, the read operation is repeated fourtimes. Read digital data is digital-to-analog converted, and theharmonic overtone component S47 four times in frequency the originallow-frequency component S46 is obtained.

Even when the low-frequency component S16 is lower in frequency than theresonance frequency f0 of the loudspeaker 33, the pitch shifter 17converts the low-frequency component S16 into the harmonic overtonecomponent S17 higher in frequency than the resonance frequency f0 of theloudspeaker 33. The harmonic overtone component S17 is added to themiddle to high-frequency component S12 and the resulting component issupplied to the loudspeaker 33. Even the small loudspeaker 33 canprovide a low frequency sound effect.

Since the harmonic overtone component S17 contained in the low-frequencycomponent S23 dynamically changes in level as shown in FIG. 3, a crisplow frequency sound effect is provided while distortion effect isrestricted.

The low-frequency component S16 is frequency-doubled orfrequency-quadrupled so that the resulting harmonic overtone componentsfall within the frequency range from the resonance frequency f0 to thefrequency upper limit f1. As a result, no unpleasant hearing impressionis caused.

For example, since a harmonic overtone component with the frequencythereof three times the fundamental component has no octavalrelationship with the fundamental component, an unpleasant hearingimpression is given to the listener. The harmonic overtone componentstwice or four times in frequency the fundamental frequency have oneoctave or two octaves higher than the fundamental frequency and cause nounpleasant hearing impression to the listener.

In the above embodiments, the level detector 24 detects the peak levelV22 within the one period Tx, and controls gain within the one periodTx. Alternatively, a mean level may be detected within the one periodTx, and gain is controlled within the one period Tx. Alternatively, thelevel of the harmonic overtone component supplied to the detector 24 isdetected for each sample, in other words, the envelope of the harmonicovertone component is detected and used to control gain. Alternatively,gain may be controlled taking into consideration the characteristics ofthe loudspeaker 33 such as attack time and release time.

The control characteristics of the gain controller 23 of FIG. 4 may beinverted. More specifically, gain is set to be smaller than 1 withrelationship V22>VTH held, and gain is set to be 1 with relationshipV22=VTH held. Gain may be set to be larger than 1 with relationshipV22<VTH held. In such a case, automatic gain control (AGC) also worksand the low frequency sound effect is thus provided.

The process of obtaining intermediate signals and the resulting audiosignal S31 from the audio signal S11 may be a digital process performedby a digital signal processor or other dedicated hardware. In such acase, the buffer memory may be shared with the digital signal process,for example.

In the above discussion, the pitch shifters 17 and 47 frequencymultiplies the input digital data D16 (S16) every period of the digitaldata D16. Alternatively, the digital data D16 may be doubled infrequency every predetermined duration of time. In such a case, an endpoint of one period and a start point of the next period may beconcatenated in a smooth fashion.

The low-frequency cutoff filter 31 may be arranged between the harmonicovertone adder 10 and the equalizer 20. The resonance frequency f0 isthe resonance frequency of the loudspeaker 33. In actual products,however, another frequency may be set as the frequency f0 taking intoconsideration a frequency at which a low frequency sound effect isdesired.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. An audio signal processing apparatus comprising: a harmonic overtone adder and an equalizer; the harmonic overtone adder including a high-pass filter, a first filter, an harmonic overtone generator, and a first combining unit; and the equalizer including a second filter, a level detector, a gain controller, and a second combining unit, wherein the high-pass filter is for extracting from an input signal a first frequency component equal to or higher than a first predetermined frequency (f0), and for supplying the first frequency component to the first combining unit, the first filter is for extracting from the input signal a second frequency component equal to or lower than f0, and for supplying the second frequency component to the harmonic overtone generator, the harmonic overtone generator is for generating a frequency-doubled harmonic overtone component from the second frequency component, and for supplying the frequency-doubled harmonic overtone component to the first combining unit, the first combining unit is for combining at least the first frequency component and the frequency-doubled harmonic overtone component to produce a first signal, and for supplying the first signal to the equalizer, the second filter is for extracting from the first signal a third frequency component, and for supplying the third frequency component to the gain controller, the level detector is for detecting a level of one of the first signal and the third frequency component, and for supplying a detection signal to the gain controller, the gain controller is for generating a fourth frequency component from the third frequency component, the fourth frequency component and the third frequency component having similar frequency characteristics, dynamically controlling a level of the fourth frequency component based on the detection signal, and supplying the fourth frequency component to the second combining unit, and the second combining unit is for combining the first signal with the fourth frequency component, and for supplying an output signal.
 2. The audio signal processing apparatus according to claim 1, wherein f0 is a resonance frequency of a loudspeaker.
 3. The audio signal processing apparatus according to claim 1, further comprising a low frequency cutoff filter for removing from the output signal a low-frequency component lower than f0.
 4. The audio signal processing apparatus according to claim 1, wherein the level detector is for determining one cycle in response to a change in a polarity of the third frequency component, and for detecting the level of the one of the first signal and the third frequency component within the one cycle.
 5. The audio signal processing apparatus according to claim 4, wherein the one cycle is a period from a first reversal to a next reversal of the polarity of the level of the third frequency component, the first reversal of the polarity being from a first state to a second state, the next reversal of the polarity also being from the first state to the second state.
 6. The audio signal processing apparatus according to claim 4, wherein the level detector detects a peak level in the one cycle.
 7. The audio signal processing apparatus according to claim 3, wherein the level detector detects a mean level throughout the one cycle.
 8. The audio signal processing apparatus according to claim 1, wherein: the first filter is a low-pass filter, the second filter is a low-pass filter, the third frequency component is equal to or lower than a second predetermined frequency (f1), and f1 is an upper frequency limit below which a signal obtained by frequency multiplying a fundamental frequency signal causes no unpleasant hearing impression.
 9. The audio signal processing apparatus according to claim 1, wherein: the first combining unit is for combining the first frequency component, the frequency-doubled harmonic overtone component, and the second frequency component to produce the first signal, the harmonic overtone generator is for supplying the frequency-doubled harmonic overtone component to the first combining unit via a first attenuator, and the first filter is for supplying the second the second frequency component to the first combining unit via a second attenuator.
 10. An audio signal processing apparatus comprising: a harmonic overtone adder and an equalizer; the harmonic overtone adder including a high-pass filter, a low-pass filter, a first band-pas filter, a frequency-doubling harmonic overtone generator, and a first combining unit; and the equalizer including a second band-pass filter, a level detector, a gain controller, and a second combining unit, wherein the high-pass filter is for extracting from an input signal a first frequency component equal to or higher than a first predetermined frequency (f0), and for supplying the first frequency component to the first combining unit, the low-pass filter is for extracting from the input signal a second frequency component equal to or lower than f0, and for supplying the second frequency component to the first combining unit, the first band-pass filter is for extracting from the input signal a third frequency component falling within a frequency band of 0.5* f0 to 0.5* a second predetermined frequency (f1), and for supplying the third frequency component to the frequency-doubling harmonic overtone generator, the frequency-doubling harmonic overtone generator is for generating a frequency-doubled harmonic overtone component from the third frequency component, and for supplying the frequency-doubled harmonic overtone component to the first combining unit, the first combining unit is for combining at least the first frequency component, the second frequency component, and the frequency-doubled harmonic overtone component to produce a first signal, and for supplying the first signal to the equalizer, the second band-pass filter is for extracting from the first signal a fourth frequency component falling within a frequency band of f0 to f1, and for supplying the fourth frequency component to the gain controller, the level detector is for detecting a level of one of the first signal and the fourth frequency component, and for supplying a detection signal to the gain controller, the gain controller is for generating a fifth frequency component from the fourth frequency component, the fifth frequency component and the fourth frequency component having similar frequency characteristics, dynamically controlling a level of the fifth frequency component based on the detection signal, and supplying the fifth frequency component to the second combining unit, and the second combining unit is for combining the first signal with the fifth frequency component to produce an output signal.
 11. The audio signal processing apparatus according to claim 10, wherein: f0 is a resonance frequency of a loudspeaker, and f1 is an upper frequency limit below which a signal obtained by frequency multiplying a fundamental frequency signal causes no unpleasant hearing impression.
 12. The audio signal processing apparatus according to claim 10, wherein: the harmonic overtone adder further includes a third band-pass filter and a frequency-quadrupling harmonic overtone generator, the third band-pass filter is for extracting from the first signal a sixth frequency component falling within a frequency band of 0.25*f0 to 0.25*f1, and for supplying the sixth frequency component to the frequency-quadrupling harmonic overtone generator, the frequency-quadrupling harmonic overtone generator is for generating a frequency-quadrupled harmonic overtone component from the sixth frequency component, and for supplying the frequency-quadrupled harmonic overtone component to the first combining unit, and combining at least the first frequency component, the second frequency component, and the frequency-doubled harmonic overtone component to produce the first signal comprises combining the first frequency component, the second frequency component, the frequency-doubled harmonic overtone component, and the frequency-doubled harmonic overtone component to produce the first signal.
 13. The audio signal processing apparatus according to claim 12, wherein: the frequency-doubling harmonic overtone generator is for supplying the frequency-doubled harmonic overtone component to the first combining unit via a first attenuator, a third combining unit, and a second low-pass filter, the frequency-quadrupling harmonic overtone generator is for supplying the frequency-quadrupled harmonic overtone component to the first combining unit via a second attenuator, the third combining unit, and the second low-pass filter, the first and second attenuators are for attenuating the frequency-doubled and frequency-quadrupled harmonic overtone components, respectively, and for supplying the attenuated frequency components to the third combining unit, the third combining unit is for combining the attenuated frequency components to produce a second signal, and for supplying the second signal to the second low-pass filter, the second low-pass filter is for extracting from the second signal a seventh frequency component equal to or lower than f1, and for supplying the seventh frequency component to the first combining unit, and combining at least the first frequency component, the second frequency component, and the frequency-doubled harmonic overtone component to produce the first signal comprises combining the first frequency component, the second frequency component, and the seventh frequency component to produce the first signal.
 14. An audio signal processing method comprising: extracting from an input signal a first frequency component equal to or higher than a first predetermined frequency (f0); extracting from the input signal a second frequency component equal to or lower than f0; generating a frequency-doubled harmonic overtone component from the second frequency component; combining at least the first frequency component and the frequency-doubled harmonic overtone component to produce a first signal; extracting from the first signal a third frequency component equal to or lower than a second predetermined frequency (f1); detecting a level of one of the first signal and the third frequency component, and supplying a detection signal based on the detected level; generating a fourth frequency component from the third frequency component, the fourth frequency component and the third frequency component having similar frequency characteristics; dynamically controlling a level of the fourth frequency component based on the detection signal; combining the first signal with the fourth frequency component. 